When we look in a rear-view mirror the number plate of the car behind appears
back to front but not upside down. Why is this?

Paul Yates, a physicist at Otago University, responded.

The odd appearance of writing as seen in a mirror is often explained as the mirror
reversing left and right. So why not top and bottom as well? In front of a mirror
hold one hand above your head and the other pointing downwards. The hand
on top is still on top in your image. Top and bottom are not reversed. Now hold
one hand to your left (call it west) and the other to the right (east). Which way do
the hands of the mirror image point?

The mirror does not reverse left and right. East maps to east in the image, west to west,
top to top, and bottom to bottom. So what is the mirror doing with letters? Try this
experiment: write your name on a piece of transparent plastic (a CD case works well)
and hold it in front of you facing the mirror. This text is not reversed in the mirror!
Move the writing further away from the mirror so it is behind you - the image still reads
as normal, but you must turn around and view it from behind, and the text now appears
reversed. Front and back have been exchanged rather than left and right.

You can also see this reversal when you point towards the mirror. The image points back
the other way towards you, inverting front and back.